CN1125495C - Negative electrode active material for lithium secondary battery and manufacturing method - Google Patents

Abstract

The present invention provides a negative electrode active material for lithium secondary batteries capable of simultaneously satisfying high capacity density, low energy consumption during production, and easy quality control, and a production method thereof. The invention relates to a new negative electrode active material for lithium secondary batteries composed of tin oxyhydroxide or tin and its composite oxyhydroxide and a manufacturing method thereof.

Description

Negative electrode active material for lithium secondary battery and preparation method thereof

The invention relates to a negative electrode active material for a lithium secondary battery and a manufacturing method thereof.

The lithium secondary electrode is generally a positive electrode composed of a compound that can occlude and release lithium ions, a negative electrode composed of metal lithium, a lithium alloy that can occlude and release lithium ions, and an organic electrolyte that dissolves lithium salt in an organic solvent. or polymer electrolyte, showing extremely high voltage and high energy density.

As the positive electrode active material, the compounds that can store and release lithium ions are known, such as lithium cobaltate (LiCoO ₂ ), lithium nickelate (LiNiO ₂ ), spinel lithium manganese oxide (LiMn ₂ O ₄ ), vanadium oxide (V ₂ O ₅ , V ₃ O ₁₁ ), etc.

As the negative electrode active material, the above-mentioned substances are well known, but in addition, for example, lithium titanium spinel oxide (LiTi ₂ O ₄ or Li _4/3 Ti _5/3 O ₄ ) (T.Ohjuku et al, JECS, 142, 1431 (1995), oxides mainly composed of tin (JP-A-7-12274, JP-7-201318, JP-7-288123), oxides mainly of silicon (JP-6-325765, EP0615296A1) and other metal oxides capable of occluding and releasing lithium.

Lithium-ion secondary batteries are classified into two types by combining the positive electrode and the negative electrode when the battery is first assembled. One is a combination of a lithium-free positive electrode and a negative electrode made of metallic lithium and a metal-containing substance (charged state), and the other is a combination of a lithium-containing positive electrode and a lithium-free negative electrode ( uncharged state). A typical example of the former is a combination of a positive electrode made of vanadium oxide and a negative electrode made of metal lithium. At this time, the following charge and discharge reactions occur, and when the battery is used, it is first discharged.

A representative example of the latter is a combination of a positive electrode made of lithium cobaltate (LiCoO ₂ ) and a negative electrode made of graphite (C). At this time, the charging and discharging reaction proceeds according to the following formula, but after the battery is assembled, the charging reaction proceeds first.

In batteries using metal lithium of formula (1) and formula (2), when charge and discharge are repeated, due to the electrolysis of so-called dendrite metal lithium, it is easy to short circuit, resulting in abnormal heat generation and battery internal pressure rise. High risk, so it has not yet reached the stage of practical use. Even if metal lithium is replaced by lithium alloy, this situation cannot be changed.

On the contrary, when the batteries of formula (3) and formula (4) are used, since lithium is occluded and released in the active material in the form of ions (thus, this type of battery is called a lithium-ion secondary battery), it can It avoids the precipitation of metal lithium, and the charge and discharge cycle life is also long, ensuring high safety. Therefore, at present, the lithium ion secondary battery can be widely used for practical use.

As the active material of the positive electrode and the negative electrode, it is desirable to have a large capacity per unit weight or unit volume, and a high battery voltage (based on Li/Li ⁺ , a higher value for the positive electrode and a lower value for the negative electrode).

As the negative electrode active material, graphite or amorphous carbon has been practically used at present, but for the former, for 6 moles of carbon atoms, a maximum of 1 mole of lithium is intercalated (LiC ₆ ), and its theoretical capacity density is 372mAh/g while for the latter It is not necessarily fully understood theoretically, but it has been reported that a capacity density of 500-600mAh/g can be obtained. However, from the point of view of discharge voltage, graphite shows a flat potential of about 0.1V (for Li/Li ⁺ ), and in the case of amorphous carbon, as the discharge progresses, its potential gradually increases, on average, about 0.5V , the battery voltage is also relatively low.

On the other hand, from the viewpoint of the amount of energy consumed in the production of these carbon materials, for graphite, it is produced at a temperature above 2000 ° C, and for amorphous carbon, it is produced at a relatively high temperature of 800 to 1000 ° C, so the energy consumption The amount of both is quite large, but extremely large especially for graphite. In addition, from the point of view of quality control in the manufacture of these carbon materials, for graphite, because of its good crystallization, the crystal lattice constant can be precisely measured by a relatively simple method, that is, powder X-ray diffraction method, and its quality control is easy. In the case of amorphous carbon, since the diffraction peaks are not clearly visible due to powder X-ray diffraction, it is difficult to control its quality.

In this way, even if the same carbon material is used, both graphite and amorphous carbon have their advantages and disadvantages respectively. As a negative active material for lithium ion batteries, which one to choose depends on which aspect to focus on among various properties. decided. In any case, if the negative electrode using carbon material is to store lithium ions in advance, it is unstable and troublesome, so it is generally assembled into the battery in an uncharged state (a state that does not store lithium ions).

Among the lithium-titanium spinel oxides mentioned above, the composition of Li _4/3 Ti _5/3 O ₄ has a capacity density of about 170mAh/g, which is small compared with carbon materials. In addition, its discharge potential is about 1.5V (for Li/Li ⁺ ) is quite high. However, since these materials contain lithium at the time of manufacture, they are suitable for forming a fully charged state at the time of battery assembly.

On the other hand, it has been reported that oxides of tin (Sn(I)O having an α-PbO ₂ structure and Sn(II)O ₂ having a rutile structure) exhibit approximately the same Capacity density (JP-A-7-235293). In addition, as its production method, in the case of SnO ₂ , it can be produced by heat-treating the precipitate obtained from the mixed aqueous solution of tin salt and alkali hydroxide at a temperature of 250°C or higher, preferably 400°C or higher. . In addition, the above-mentioned report also states that the discharge capacity density was not significantly lowered for a charge-discharge cycle of 10 cycles, but it does not describe the test results of charge-discharge cycles beyond that.

As mentioned above, there are various negative electrode active material materials for lithium secondary batteries, but each has its own advantages and disadvantages in terms of its characteristics and manufacturing process, and not all of them have advantages. In other words, as an evaluation criterion of the negative electrode active material material, its selection method is different according to its requirements. In the present invention, the three points of high capacity density, low energy consumption during production, and easy quality control are preferably considered as evaluation criteria, and an object of the present invention is to provide a more excellent negative electrode active material than conventional ones.

The present invention achieves the above object by using tin oxyhydroxide as a new negative electrode active material.

Fig. 1 is the typical X-ray diffraction figure of oxytin hydroxide of the present invention.

Fig. 2 is an X-ray diffraction pattern of a tin oxyhydroxide system according to an example of the present invention.

Fig. 3 is the charging characteristics of an embodiment of the lithium secondary battery negative electrode using the negative electrode active material of the present invention.

Fig. 4 is a discharge curve of an embodiment of a negative electrode of a lithium secondary battery using negative active materials according to the present invention.

For oxytin hydroxide, the oxytin hydroxide represented by chemical formula Sn(I) ₃ (OOH) ₂ or Sn(I) ₃ O ₂ (OH) ₂ and the oxytin hydroxide represented by Sn(II) ₃ (OOH) ₄ or Sn(II) ₃ O ₄ (OH) ₄ represents a high-tin oxyhydroxide. The inventors of the present application have found that when these materials are used as negative electrode active materials of lithium-ion batteries, they show about 800 mAh/g of amorphous carbon in the discharge potential range of 0.2 to 1.5 V (vs. Li/Li ⁺ ). Or the same or higher capacity density than tin oxide.

Oxytin hydroxide, initially from charging. In this charge process, lithium is occluded, and lithium is released in the next discharge process. Generally, in the first charging process, absorb lithium equivalent to an amount of 100mAh/g or more, and for the next discharge process, only discharge lithium equivalent to an amount of about 800mAh/g, but after the second charging process , the charging and discharging capacity is close. There are still many unknowns about the reaction mechanism of the charging and discharging process of this substance, but when the potential transition is observed, it is at least higher than the potential of metal lithium electrolysis (OV, for Li/Li ⁺ ), so it can be confirmed that during the charging process, Lithium is occluded, and lithium is released during discharge.

Oxytin hydroxide, generally the precipitate obtained by adding an aqueous solution of alkaline hydroxide to an aqueous solution of tin salt or an organic solvent solution such as ethanol, is filtered and washed, and dried at a temperature below 150°C , obtained by heat treatment. A negative electrode active material for lithium ion secondary batteries obtained at a low temperature of 150° C. or lower has not been found so far. From the viewpoint of low energy consumption, it can be said that this active material is excellent. In addition, oxy-stannous hydroxide has excellent crystallinity and can obtain a very clear X-ray diffraction pattern, which is extremely advantageous in quality management, only by drying and heat-treating at such a low temperature.

It goes without saying that the aforementioned tin oxide and the tin oxyhydroxide of the present invention are completely different substances.

In addition, one of the manufacturing methods of lithium and tin composite oxides of the Li _x SnO _y structure disclosed in Japanese Patent Laid-Open No. 7-20318 as the negative active material of the lithium secondary battery is to oxidize the lithium salt and the oxyhydroxide A method of solid phase sintering of tin mixtures. At this time, Li _x SnO _y is different from tin oxide and is used as a negative electrode active material in the charged state. High tin oxyhydroxide is always just the initial raw material for manufacturing Li _x SnO _y , and high tin oxyhydroxide itself There is no suggestion that it can become a negative electrode active material. Furthermore, there can be said to be no mention at all of stannous oxyhydroxide. To repeat, it was not known at all that tin oxyhydroxide effectively functions as a negative electrode active material incorporated in a battery in an uncharged state, but was discovered by the inventors of the present application.

When producing stannous oxyhydroxide in tin oxyhydroxide, it is typically desired to dissolve stannous chloride (SnCl ₂ ) in water dissolved with a very small amount of hydrochloric acid, and then add ammonia water to precipitate it. The precipitate was filtered, washed, and then dried at 80° C. to remove water. The reaction is believed to proceed as follows. --------(5)

The obtained stannous oxyhydroxide Sn(I) ₃ (OOH) ₂ shows the X-ray diffraction pattern shown in Fig. 1 when pure, but sometimes contains unknown impurities. In this way, even tin oxyhydroxide containing some impurities can sufficiently function as a negative electrode active material.

Among the raw materials for producing stannous oxyhydroxide, the tin compound is not limited to the aforementioned stannous chloride, and other inorganic salts and organic acid salts of stannous may be used. In addition, other alkaline hydroxide aqueous solutions may be used instead of ammonia water. Furthermore, not only water but also organic solvents such as methanol, or water and an organic solvent are also effective as the solvent of the stannous salt and the alkali hydroxide. On the other hand, in many cases, if a small amount of acid is added to a stannous salt that is difficult to dissolve in a solvent, it can be easily dissolved. As the acid, besides hydrochloric acid, inorganic acids such as nitric acid and sulfuric acid, and organic acids such as acetic acid can be used.

The drying heat treatment temperature of stannous oxyhydroxide as a precipitate is about 80° C. as described above, but a higher temperature may be used as long as stannous oxyhydroxide does not decompose. The high-valent tin oxyhydroxide may be used as a raw material of the tin salt as long as a high-valent tin salt is used.

In the above tin oxyhydroxide, it is also effective to replace a part of tin with other elements. As substituting elements, Mg, Ca, Ni, Mn, V, Ti, Pb, Al, Ge, As, Si, and Sb are effective.

The above-mentioned tin oxyhydroxide has low electronic conductivity, so when it is made into a negative electrode, it can be mixed with conductive powder of carbon, and coated on the electrode substrate together with an appropriate amount of binder. Also, it is most suitable to incorporate a negative electrode using tin oxyhydroxide as an active material in combination with a positive electrode using a positive electrode active material containing lithium (for example, LiCO ₂ ).

However, the tin hydroxide negative electrode is used as the counter electrode to absorb metal lithium, lithium alloy, and lithium intercalation material, and in another electrolytic cell with an organic electrolyte solution, through prior electrolytic reduction, it is made to absorb in the negative electrode. stay lithium. The negative electrode of oxytin hydroxide for storing lithium can be combined with a positive electrode not containing lithium (such as V ₂ O5 ) and loaded into a battery.

Example 1

After dissolving 5g of stannous chloride in 2% aqueous nitric acid solution and stirring, add 40ml of ammonia water to obtain a precipitate. This precipitate was filtered, washed with water, and dried at 80°C. The powder X-ray diffraction pattern of the obtained substance is shown in FIG. 2 . From all the diffraction peaks, the substance is Sn ₃ O ₂ (OH) ₂ or Sn ₃ (OOH) ₂ oxytin hydroxide. In addition, in Figure 2, the peak with 2 *marks seen from the low angle side can be presumed to be caused by impurities, but it is not yet clear that this substance belongs to the space group P4/mnc, and its unit lattice The constants are a = 7.89 Angstroms and C = 9.01 Angstroms.

Next, mix 77% of the oxytin hydroxide powder with 9% of acetylene black as a conductive material, and 14% of a polyvinylidene fluoride-methylpyrrolidone solution as a binder, coat it on a stainless steel mesh, and extrude After that, as the negative pole. This negative electrode is immersed in the electrolytic solution composed of 1 mol% _LiClO mixed organic solvent (ethylene carbonate, diethyl carbonate, dimethyl carbonate) solution together as the metal lithium plate as the opposite pole, for Charge and discharge test. The test is under the current density of 0.5mA/cm ² , first charge and then discharge. The charging characteristics of the negative electrode in the first cycle and the second cycle are shown in FIG. 3 , and the discharging characteristics are shown in FIG. 4 . It was shown that the discharged electric quantity was relatively small relative to the charged electric quantity, and a relatively large value of 700 mAh/g was obtained for the discharged electric quantity.

Example 2

In Example 1, stannous acetate was dissolved in nitric acid acidic aqueous solution to replace stannous chloride, and then ammonia water was added to obtain oxytin hydroxide with a discharge capacity of 800mAh/g.

Example 3

In Example 1, tin protochloride was dissolved in ethanol to replace water, and then, an ethanol solution of ammonia was added to obtain oxytin protohydroxide with a discharge capacity of 780mAh/g.

Example 4

In Example 1, tin protochloride and magnesium chloride were dissolved in 2% hydrochloric acid aqueous solution, and an aqueous solution of sodium hydroxide was added to obtain a composite oxyhydroxide of Sn _2.8 Mg _0.2 (OOH) ₂ . This material showed a discharge capacity density of about 500 mAh/g as a negative electrode active material.

As described above, the present invention provides a negative electrode active material for lithium secondary batteries excellent in high capacity density, low energy consumption during production, and high crystallinity, and its industrial value is extremely great.

Claims (8)

1. A negative electrode active material for a lithium secondary battery, characterized by consisting of tin oxyhydroxide.

2. A negative electrode active material for a lithium secondary battery, characterized in that the material is composed of a tin-containing composite oxyhydroxide in which a part of tin is substituted by an element selected from the groupconsisting of Mg, Ca, Ni, Mn, V, Ti, Pb, Al, Ge, As, Si and Sb.

3. The negative active material for a lithium secondary battery as claimed in claim 1, wherein the tin oxyhydroxide is stannous oxyhydroxide.

4. The negative active material for a lithium secondary battery as claimed in claim 1, wherein the tin oxyhydroxide is high tin oxyhydroxide.

5. A method for producing a negative electrode active material for a lithium secondary battery, characterized in that tin salt is dissolved in water in which an acid is dissolved, an organic solvent or a mixed solution of water and an organic solvent, and then an aqueous solution of an alkaline hydroxide is added to the solution to produce tin oxyhydroxide.

6. The process according to claim 5, wherein the alkali hydroxide is ammonium hydroxide.

7. A method for producing a negative electrode active material for a lithium secondary battery, characterized in that a tin salt and a salt of an element selected from the group consisting of Mg, Ca, Ni, Mn, V, Ti, Pb, Al, Ge, As, Si and Sb are dissolved in water in which an acid is dissolved, an organic solvent or a mixed solution of water and an organic solvent, and then an aqueous solution of an alkaline hydroxide is added to the resulting solution to produce a tin-containing composite oxyhydroxide.

8. The method of claim 7, wherein said alkaline hydroxide is ammonium hydroxide.

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